LiDAR (light detection and ranging) uses laser technology to make precise distance measurements over short or long distances. LiDAR units have found widespread application in both industry and the research community.
The predecessor technology to current LiDAR units were object detection systems that could sense the presence or absence of objects within the field of view of one or more light beams based on phase shift analysis of the reflected light beam. Examples of these kinds of object detection systems in the field of vehicle “blind spot” warning systems include U.S. Pat. Nos. 5,122,796, 5,418,359, 5,831,551, 6,150,956, and 6,377,167.
Current LiDAR units are typically scanning-type units that emit beams of light in rapid succession, scanning across the angular range of the unit in a fan-like pattern. Using a time of flight calculation applied to any reflections received, instead of just a phase shift analysis, the LiDAR unit can obtain range measurements and intensity values along the singular angular dimension of the scanned beam. LiDAR units typically create the scanning beam by reflecting a pulsed source of laser light from a rotating mirror. The mirror also reflects any incoming reflections to the receiving optics and detector(s).
Single-axis-scan LiDAR units will typically use a polygonal mirror and a pulsed laser source to emit a sequence of light pulses at varying angles throughout the linear field of view. Return signals are measured by a bandpass photoreceptor that detects the wavelength of light emitted by the laser. The field of view of the photoreceptor covers the entire one-dimensional scan area of the laser. Thus, each subsequent emitted pulse of laser light must occur only after the reflected signal has been received for the previous laser pulse. Dual-axis-scan LiDAR units produce distance-measured points in two dimensions by using, for instance, a pair of polygonal mirrors. The horizontal scan mirror rotates at a faster rate than the vertical scan mirror. An example of a long-range scanning-type LiDAR for satellite and aircraft is U.S. Pat. No. 7,248,342 that describes scanning of both the transmitted and received laser signals along with a linear arrangement of pixel sensors referred to as a “push broom” sensor for detecting the received laser signals as it is scanned back and forth. U.S. Pat. No. 8,599,367 describes an improved push broom approach that uses laser light with different frequency components and then separates the frequency components received as reflected signals to be detected by different linear pixel sensors.
Image-type LiDAR units offer a way to acquire a 3D map of a scene via a solid state or mostly solid state approach in the form of a detector array. These image-type devices are often referred to as flash LiDAR devices because they illuminate an entire 2D field of view with a blanket of light and then simultaneously measure the return value time for each photoreceptor location in the detector array that covers the field of view. Examples of image-type LiDAR units include U.S. Pat. Nos. 7,551,771 and 8,072,581. Unfortunately, these approaches have been relegated to very close proximity applications due to the low incident laser power available for each location in the field of view. For flash LiDAR at longer ranges, the usable field of view is typically too small for applications like autonomous vehicle navigation without the use of high performance cameras operating in the picosecond range for exposure times.
U.S. Pat. No. 7,969,558 describes a LiDAR device that uses multiple lasers and a 360-degree scan to create a 360-degree 3D point cloud for use in vehicle navigation. The disclosed system has three limitations. First, the rotating scan head makes the unit impractical for widespread use on autonomous vehicles and makes it unusable for inclusion in mobile devices like smart phones, wearable devices, smart glasses, etc. Second, multiple units cannot work effectively in the same relative physical space due to the potential of crosstalk. Third, the throughput of the device is limited to measurements along a single angular direction for each emitted light pulse.
U.S. Publ. Appl. No. 2011/0313722 A1 describes a LiDAR technique used for determining the object distance in the presence of an aerosol cloud. The technique relies on analysis of a trailing edge of a given light pulse compared to an established threshold to remove reflected energy from the aerosol cloud, such as fog. U.S. Pat. No. 8,242,428 describes a LiDAR system that utilizes modulated light pulses detected by two detectors at different distances to perform quantum ghost imaging analysis. While these techniques may be useful for the specific issues addressed, such as fog or ghost imaging, these techniques address special case scenarios and are not generally applicable beyond these special cases.
LiDAR units have the potential to be utilized extensively in applications like autonomous vehicle navigation, robotics, mobile computing, wearable devices, and collision avoidance systems for unmanned aerial vehicles (UAVs), as well as high precision image analysis such as surface defect detection and texture analysis, as well as detection of roadway conditions and contaminations. Low and medium resolution LiDAR systems are typically used for object detection and some limited object identification. By increasing the precision of LiDAR processing, object identification can be markedly increased. However, the high throughput and high resolution necessary for autonomous vehicle navigation and other high precisions image analysis cannot be met with present LiDAR approaches. Furthermore, 3D point cloud approaches are inadequate for object identification and high-frequency feature extraction in real-time applications like autonomous vehicle navigation.